Flue gas treatment system and method

ABSTRACT

A wet desulfurization apparatus which removes sulfur oxides in flue gas from a boiler 11 includes a mist collection/agglomeration apparatus which is provided on a downstream side of the desulfurization apparatus and forms agglomerated SO3 mist by causing particles of SO3 mist contained in flue gas 12B from the wet desulfurization apparatus to be bonded together and have bloated particle sizes; a CO2 recovery apparatus constituted by a CO2 absorption tower having a CO2 absorption unit which removes CO2 contained in flue gas by being brought into contact with a CO2 absorbent and an absorbent regeneration tower which recovers CO2 by releasing CO2 from the CO2 absorbent having absorbed CO2 and regenerates the CO2 absorbent; and a mist collection unit which collects CO2 absorbent bloated mist bloated by the CO2 absorbent being absorbed by the agglomerated SO3 mist in the CO2 absorption unit.

TECHNICAL FIELD

The present invention relates to a flue gas treatment system and a method in which the discharge of an amine absorbent to the outside of a system is significantly suppressed when CO₂ in flue gas is absorbed and removed by the amine absorbent.

BACKGROUND ART

In recent years, as one of causes of the global warming phenomenon, the greenhouse effect by CO₂ has been pointed out, and countermeasures to protect the global environment have become urgent tasks internationally. CO₂ generation sources include all human activity fields in which fossil fuels are burned, and there is a tendency for the demand for suppression of the discharge to be further strengthened. Accordingly, for power generation facilities such as thermal power plants that use large amounts of fossil fuels, methods of removing and recovering CO₂ in combustion flue gas by bringing the combustion flue gas in industrial facilities such as a boiler or gas turbine into contact with an amine-based CO₂ absorbent, and flue gas treatment systems for storing the recovered CO₂ without release to the air have been studied vigorously.

As a process for removing and recovering CO₂ from the combustion flue gas by using the above-mentioned amine-based CO₂ absorbent, a CO₂ recovery apparatus having a process for bringing combustion flue gas and the CO₂ absorbent into contact with each other in a CO₂ absorption tower (hereinafter, also simply referred to as “absorption tower”), and a process for heating the CO₂ absorbent having absorbed CO₂ in a CO₂ absorbent regeneration tower (hereinafter, also simply referred to as “regeneration tower”) to cause CO₂ to diffuse, regenerating the CO₂ absorbent to be returned to the CO₂ absorption tower for reuse is suggested (for example, refer to PTL 1).

In the CO₂ absorption tower, CO₂ in the flue gas is absorbed by the CO₂ absorbent during countercurrent contact through a chemical reaction (exothermic reaction) by using the amine-based CO₂ absorbent such as alkanolamines, and the flue gas from which CO₂ has been removed is released to the outside of the system. The CO₂ absorbent having absorbed CO₂ is also called a rich solution. The rich solution is pressurized by a pump, is heated in a heat exchanger by a high-temperature CO₂ absorbent (lean solution) which is regenerated by the diffuse of CO₂ in the regeneration tower, and is supplied to the regeneration tower.

In the CO₂ recovery process using the CO₂ absorbent, CO₂-removed flue gas from which CO₂ has been removed from the combustion flue gas is released to the air. However, since an extremely small portion of a small amount of the amine absorbent is present in the released gas, it is necessary to reduce the amount thereof discharged.

Particularly in the future, when regulations on CO₂ removal are started, there is a possibility that a CO₂ removal facility itself may increase in size, and it is necessary to further reduce the amount thereof discharged.

As a technique for preventing the release of the amine absorbent, for example, a method in which a plurality of stages of washing units are provided on the downstream side of a CO₂ absorption unit of a CO₂ absorption tower of a CO₂ removal apparatus to bring CO₂-removed flue gas and washing water into gas-liquid contact with each other, thereby recovering an amine compound entrained in the CO₂-removed flue gas (PTL 2) is suggested.

In addition, as another technique, a method in which sulfuric acid is sprayed from a sulfuric acid spraying device toward CO₂-removed flue gas discharged from a CO₂ absorption tower to cause an amine absorbent entrained in the discharged gas to become a basic amine compound sulfate, and a basic amine compound in gas discharged from a decarbonator that collects the basic amine compound sulfate is recovered, is suggested (PTL 3).

Furthermore, in the flue gas treatment system, there is a problem that in a case where mist generating substances which are generation sources of mist generated in an absorption tower in a CO₂ recovery apparatus are contained in flue gas introduced into a CO₂ absorption tower, that absorbs CO₂, in the CO₂ recovery apparatus, a CO₂ absorbent contains the mist generating substances entrained therein, and thus the amount of the CO₂ absorbent scattering toward the outside of the system increases. Therefore, countermeasures against this have been studied (PTL 4).

CITATION LIST Patent Literature

[PTL 1] Japanese Unexamined Patent Application Publication No. 3-193116

[PTL 2] Japanese Unexamined Patent Application Publication No. 2002-126439

[PTL 3] Japanese Unexamined Patent Application Publication No. 10-33938

[PTL 4] International Publication No. W02011/152547

SUMMARY OF INVENTION Technical Problem

However, in the above-described suggestions, although the gaseous amine absorbent in the CO₂-removed flue gas released from the CO₂ absorption tower can be reduced, there is concern that the mist-like amine absorbent may be released together with the flue gas. Accordingly, this needs to be suppressed.

When the mist-like amine absorbent is released to the outside of the system, there is a problem that the amount of the CO₂ absorbent that can be originally used for CO₂ removal in the CO₂ absorption tower is reduced and needs to be replenished. Therefore, it is desired to establish a flue gas treatment system and a method capable of further suppressing the release of amine compounds entrained in the flue gas.

An object of the present invention is to provide a flue gas treatment system and a method capable of significantly suppressing entrainment of a CO₂ absorbent when treated flue gas from which CO₂ has been removed is discharged to the outside of a system.

Solution to Problem

A first invention of the present invention to solve the problems is a flue gas treatment system including: a desulfurization apparatus which removes sulfur oxides in flue gas from a boiler; a mist collection/agglomeration apparatus which is provided on a downstream side of the desulfurization apparatus and forms agglomerated and bloated mist by causing particles of mist contained in the flue gas to be bonded together and have bloated particle sizes; a CO₂ recovery apparatus constituted by a CO₂ absorption tower having a CO₂ absorption unit which removes CO₂ contained in the flue gas by being brought into contact with a CO₂ absorbent and an absorbent regeneration tower which recovers CO₂ by releasing CO₂ from the CO₂ absorbent having absorbed CO₂ and regenerates the CO₂ absorbent; and a mist collection unit which is provided on a gas flow downstream side of the CO₂ absorption unit and collects CO₂ absorbent bloated mist bloated by the CO₂ absorbent being absorbed by the agglomerated and bloated mist in the CO₂ absorption unit.

In the first invention, a second invention is the flue gas treatment system in which a flow velocity of the flue gas in the mist collection/agglomeration apparatus exceeds a critical filtration wind velocity for mist collection.

In the first or second invention, a third invention is the flue gas treatment system further including: a washing unit provided between the CO₂ absorption unit and the mist collection unit.

In any one of the first to third inventions, a fourth invention is the flue gas treatment system in which the mist collection/agglomeration apparatus causes the mist to be bloated by a wire mesh.

In any one of the first to third inventions, a fifth invention is the flue gas treatment system in which the mist collection/agglomeration apparatus causes the mist to be bloated through charging.

A sixth invention is a flue gas treatment method including: forming agglomerated and bloated mist by causing particles of mist contained in flue gas from a wet desulfurization apparatus, which removes sulfur oxides in the flue gas from a boiler, to be bonded together and have bloated particle sizes; causing the agglomerated and bloated mist to re-scatter and be introduced to a CO₂ absorption unit side which removes CO₂ by bringing a CO₂ absorbent on a gas flow downstream side into contact therewith; when CO₂ contained in the flue gas is removed by being brought into contact with the CO₂ absorbent, forming CO₂ absorbent bloated mist bloated by the CO₂ absorbent being absorbed by the agglomerated and bloated mist; and collecting the CO₂ absorbent bloated mist by a mist collection unit.

In the sixth invention, a seventh aspect is the flue gas treatment method in which a flow velocity of the flue gas containing the re-scattering agglomerated and bloated mist exceeds a critical filtration wind velocity for mist collection.

Advantageous Effects of Invention

According to the present invention, by the mist collection/agglomeration apparatus, agglomerated and bloated mist is formed by causing particles of mist contained in the flue gas from the desulfurization apparatus to be bonded together and have bloated particle sizes, the agglomerated and bloated mist is caused to re-scatter and be introduced to the CO₂ absorption unit side which removes CO₂ by bringing the CO₂ absorbent on the gas flow downstream side into contact therewith. Thereafter, when CO₂ contained in the flue gas is removed by being brought into contact with the CO₂ absorbent, CO₂ absorbent bloated mist bloated by the CO₂ absorbent being absorbed by the agglomerated and bloated mist as the nucleus is formed, and the CO₂ absorbent bloated mist can be collected by the mist collection unit. Accordingly, when the treated flue gas from which CO₂ has been removed is discharged to the outside of the system, entrainment of the CO₂ absorbent can be significantly suppressed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a flue gas treatment system according to Example 1.

FIG. 2-1 is a schematic view illustrating a mechanism for bloating mist according to the present invention.

FIG. 2-2 is a schematic view illustrating a mechanism for bloating mist according to the related art.

FIG. 3 is a schematic view of mist bloating according to the present invention.

FIG. 4 is a diagram showing an example of the relationship between a filtration wind velocity and a mist removal ratio.

FIG. 5 is a diagram showing an example of the relationship between the filtration wind velocity, a mist adhesion ratio, and a mist re-scattering ratio.

FIG. 6 is a schematic view of mist bloating caused by charging.

FIG. 7 is a diagram of the relationship between a SO₃ mist concentration in an inlet flue gas of a CO₂ absorption unit and an amine concentration in gas released from the outlet of a mist collection unit of a CO₂ absorption tower.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred examples of the present invention will be described in detail with reference to the accompanying drawings. In addition, the present invention is not limited by the examples, and in a case where there are a plurality of examples, a combination of the examples is also included.

Example 1

FIG. 1 is a schematic view of a flue gas treatment system according to Example 1. As illustrated in FIG. 1, a flue gas treatment system 10 according to this example includes: a wet desulfurization apparatus 13 which removes sulfur oxides in flue gas 12A from a boiler 11; a mist collection/agglomeration apparatus 14 which is provided on the downstream side of the desulfurization apparatus 13 and forms agglomerated SO₃ mist 51 which is agglomerated and bloated mist by causing particles of SO₃ mist 50 contained in flue gas 12D from the wet desulfurization apparatus 13 to be bonded together and have bloated particle sizes; a flue gas cooling tower 15 which cools flue gas 12C from the mist collection/agglomeration apparatus 14; a CO₂ recovery apparatus 18 constituted by a CO₂ absorption tower 16 having a CO₂ absorption unit 16A which removes CO₂ contained in flue gas 12D from the flue gas cooling tower 15 by being brought into contact with a CO₂ absorbent and an absorbent regeneration tower 17 which recovers CO₂ by releasing CO₂ from the CO₂ absorbent having absorbed CO₂ and regenerates the CO₂ absorbent; a washing unit 16B which is provided on the gas flow downstream side of the CO₂ absorption unit 16A and washes flue gas 12E; and a mist collection unit 16C which is provided on the gas flow downstream side of the washing unit 16B and collects CO₂ absorbent bloated mist 53 bloated by the CO₂ absorbent being absorbed by the agglomerated SO₃ mist 51 in the CO₂ absorption unit 16A and the washing unit 16B. In addition, the behavior of the mist (the SO₃ mist 50, the agglomerated SO₃ mist 51, and the CO₂ absorbent bloated mist 53) will be described later with reference to FIGS. 2-1 and 2-2.

Here, in this example, the SO₃ mist exemplifies the mist. However, the present invention is not limited thereto, and for example, fine mist or the like generated by causing moisture to adhere to solid particles such as fine coal ash may be exemplified.

Here, in the flue gas treatment system according to this example, CO₂ in the combustion flue gas exemplifies an object to be removed. In FIG. 1, only main facilities are illustrated, and accessory facilities are omitted. In FIG. 1, reference numeral 12G denotes a purified flue gas, 16D denotes a decarbonated combustion flue gas discharge port, 16E denotes an absorbent supply port, 16F denotes a liquid distributor, 15 a denotes a cooling water liquid distributor, 15 b denotes a cooling water circulation pump, 15 c denotes a make-up water supply line, 15 d denotes a drainage discharge line, 21 denotes a discharge pump for the absorbent having absorbed CO₂, 22 denotes a heat exchanger, 17 denotes an absorbent regeneration tower, 17 a is a liquid distributor, 23 denotes an upper filling unit, 24 denotes a reflux water pump, 25 denotes a CO₂ separator, 26 denotes a recovered CO₂ discharge line, 27 denotes a regeneration tower reflux cooler, 28 denotes a nozzle, 29 denotes a regeneration tower reflux water supply line, 30 denotes a combustion flue gas supply blower, 31 denotes a cooler, and 32 denotes a regeneration heater (reboiler).

In FIG. 1, sulfur oxides in the flue gas 12A are removed from the flue gas 12A from the boiler 11 by the wet desulfurization apparatus 13, and the flue gas 12D from the desulfurization apparatus 13 is introduced into the mist collection/agglomeration apparatus 14. Here, particles of SO₃ mist are agglomerated and bloated, thereby forming agglomerated SO₃ mist. Thereafter, the flue gas 12C containing the agglomerated SO₃ mist is pushed into the flue gas cooling tower 15 by the combustion flue gas supply blower 30, and is cooled by being brought into contact with the cooling water from the cooling water liquid distributor 15 a in a filling unit 15 e. Next, the cooled flue gas 12D is guided into the CO₂ absorption tower 16 through a combustion flue gas supply port 16 a of the CO₂ absorption tower 16. Cooling water contacting the flue gas 12C accumulates in the lower portion of the flue gas cooling tower 15 and circulates to the cooling water liquid distributor 15 a by the cooling water circulation pump 15 b so as to be used. Here, in a case where the amount of moisture in the flue gas 12B is small, the cooling water is gradually lost by humidifying and cooling the combustion flue gas. Accordingly, cooling water is replenished by the make-up water supply line 15 c. In a case where the amount of moisture in the flue gas 12B is large, moisture in the combustion flue gas condenses due to contact with the cooling water and causes an increase in the amount of the cooling water. Accordingly, excess waste water is discharged by the drainage discharge line 15 d.

Next, the flue gas 12D pushed into the CO₂ absorption tower 16 is brought into countercurrent contact with the CO₂ absorbent at a constant concentration supplied from the liquid distributor 16F in the filling unit in the CO₂ absorption unit 16A, CO₂ in the flue gas 12D is absorbed and removed by the absorbent, and the decarbonated flue gas 12E is directed to the washing unit 16B on the gas flow downstream side. The absorbent supplied to the CO₂ absorption tower 16 absorbs CO₂ and typically reaches a temperature higher than the temperature at the combustion flue gas supply port 16 a due to the heat of reaction caused by the absorption, and is sent to the heat exchanger 22 by the discharge pump 21 for the absorbent having the absorbed CO₂ so as to be heated and guided to the absorbent regeneration tower 17. Temperature control of the regenerated absorbent can be performed by the heat exchanger 22 or, if necessary, by the cooler 31 provided between the heat exchanger 22 and the absorbent supply port 16E.

In the absorbent regeneration tower 17, the absorbent is regenerated in a lower filling unit 17 b through heating by the regeneration heater (reboiler) 32, is cooled by the heat exchanger 22, and is returned to the CO₂ absorption tower 16 side. In the upper portion of the absorbent regeneration tower 17, CO₂ separated from the absorbent comes into contact with reflux water supplied from the nozzle 28 in the upper filling unit 23, is cooled by the regeneration tower reflux cooler 27, is separated from the reflux water having condensed water vapor with CO₂ entrained therein by the CO₂ separator 25, and is guided to a CO₂ recovery process by the recovered CO₂ discharge line 26.

A portion of the reflux water is refluxed to the absorbent regeneration tower 17 by the reflux water pump 24, and the portion is supplied to the regeneration tower reflux water supply port 29 a of the CO₂ absorption tower 16 via the regeneration tower reflux water supply line 29.

Since the amount of the absorbent contained in the regenerated reflux water is small, the absorbent comes into contact with the flue gas in the washing unit 16B of the CO₂ absorption tower 16 and contributes to the recovery of a small amount of absorbent contained in the decarbonated combustion flue gas 12E.

FIG. 2-1 is a schematic view illustrating a mechanism for bloating mist according to the present invention. FIG. 2-2 is a schematic view illustrating a mechanism for bloating mist according to the related art.

First, as illustrated in FIG. 2-1, in the flue gas 12A from the boiler 11 introduced into the wet desulfurization apparatus 13, the SO₃ mist 50 is generated from a portion of SO₃ gas in the desulfurization apparatus 13.

The generated SO₃ mist 50 is contained in the flue gas 12B discharged from the desulfurization apparatus 13. The SO₃ mist 50 in the flue gas 12B introduced into the mist collection/agglomeration apparatus 14 provided on the downstream side of the desulfurization apparatus 13 adheres to, for example, a wire mesh, agglomerates, and bloats to form the agglomerated SO₃ mist 51.

The flue gas 12C containing the agglomerated SO₃ mist 51 bloated in the mist collection/agglomeration apparatus 14 is then introduced into the flue gas cooling tower 15. In the flue gas cooling tower 15, the agglomerated SO₃ mist 51 absorbs water vapor 60 in the flue gas cooling tower 15 and becomes dilute sulfuric acid, thereby forming dilute sulfuric acid mist 52 with the agglomerated SO₃ mist 51 as the nucleus.

The flue gas 12D containing the dilute sulfuric acid mist 52 with the agglomerated SO₃ mist 51 bloated in the flue gas cooling tower 15 as the nucleus is then introduced into the CO₂ absorption unit 16A in the CO₂ absorption tower 16.

The dilute sulfuric acid mist 52 with the agglomerated SO₃ mist 51 in the flue gas introduced into the CO₂ absorption unit 16A as the nucleus absorbs amine vapor 61 in a case where the water vapor 60 in the CO₂ absorption unit 16A and an amine compound as the CO₂ absorbent are used, and forms the CO₂ absorbent bloated mist 53 which contains a high concentration of amine and is thus bloated.

The flue gas 12E containing the CO₂ absorbent bloated mist 53 bloated in the CO₂ absorption unit 16A is then introduced into the washing unit 16B.

In the washing unit 16B, the CO₂ absorbent bloated mist 53 which contains a high concentration of amine and is thus bloated is formed by further absorbing the water vapor 60 and the amine vapor 61 in the washing unit 16B.

A demister provided with a wire mesh is used in the mist collection unit 16C on the outlet side in the CO₂ absorption tower 16, and the bloated CO₂ absorbent bloated mist 53 is collected by the demister.

Contrary to this, in the related art, as illustrated in FIG. 2-2, the mist collection/agglomeration apparatus 14 as in this example is not provided, and thus the size of the nucleus forming dilute sulfuric acid mist in the flue gas cooling tower 15 is small. As a result, even in a case where there is bloating due to the CO₂ absorbent in the CO₂ absorption tower 16, CO₂ absorbent bloated mist 53 which is smaller than that in the case of the present invention is formed. In the mist collection unit 16C on the outlet side in the CO₂ absorption tower 16, the efficiency of the demister provided with the wire mesh in collecting the bloated CO₂ absorbent bloated mist 53 is low.

Table 1 shown below shows the outlet mist particle size of the desulfurization apparatus 13, the outlet mist particle size of the mist collection/agglomeration apparatus 14, and the inlet mist particle size of the mist collection unit 16C in the related art and Example 1.

In the related art, since the mist collection/agglomeration apparatus 14 is not installed, the SO₃ mist 50 (particle size 0.1 to 1.0 μm) at the outlet of the desulfurization apparatus 13 becomes the nucleus, is introduced into the CO₂ absorption unit 16A, and is bloated herein such that the inlet mist particle size of the mist collection unit 16C was 0.5 to 2.0 μm.

Contrary to this, in Example 1, since the mist collection/agglomeration apparatus 14 is installed, in a case where two particles of mist having an outlet mist particle size of 0.1 to 1.0 μm for the desulfurization apparatus 13 agglomerate, the outlet mist particle size of the mist collection/agglomeration apparatus 14 is bloated to become 0.12 to 1.2 μm. The agglomerated SO₃ mist 51 with the two agglomerating particles of mist becomes the nucleus, is introduced into the CO₂ absorption unit 16A, and is bloated herein such that the inlet mist particle size of the mist collection unit 16C was 0.6 to 2.3 μm.

In addition, in a case where, for example, five particles of mist having an outlet mist particle size of 0.1 to 1.0 μm for the desulfurization apparatus 13 agglomerate, the outlet mist particle size of the mist collection/agglomeration apparatus 14 is bloated to become 0.17 to 1.7 μm. The agglomerated SO₃ mist 51 with the five agglomerating particles of mist becomes the nucleus, is introduced into the CO₂ absorption unit 16A, and is bloated herein such that the inlet mist particle size of the mist collection unit 16C was 0.7 to 2.9 μm.

As a result, according to Example 1, the collection efficiency in the mist collection unit 16C could be improved.

TABLE 1 Example 1 Two particles of Five particles Related mist of mist art agglomerated agglomerated Outlet mist particle 0.1 to 1.0 μm  0.1 to 1.0 μm  0.1 to 1.0 μm size of desulfurization apparatus 13 Outlet mist particle — 0.12 to 1.2 μm 0.17 to 1.7 μm size of mist collection/agglomeration apparatus 14 Inlet mist particle size 0.5 to 2.0 μm  0.6 to 2.3 μm  0.7 to 2.9 μm of mist collection unit 16 C

The reason why the removing means using the wire mesh is selected as the mist collection/agglomeration apparatus 14 will be described below in Table 2.

In a case where mist removal is performed, hitherto, a mist removal apparatus which uses dense filter fabric such as a candle filter using the Brownian diffusion principle is suitable to remove fine mist of several micrometers or smaller. However, it is necessary to perform an operation at a low filtration wind velocity, resulting in an increase in the size of the apparatus, which is not preferable.

In addition, in a case where a corrugated mist removal apparatus is used, it is possible to perform an operation at a high filtration wind velocity, and thus a reduction in the size of the apparatus is possible. However, the inertial forces of fine particles decrease in proportion to mass, and the mist removal efficiency decreases. Therefore, it is not preferable in practice to use the corrugated mist removal apparatus for removing fine particles.

Therefore, the removing means using the wire mesh is preferable as the mist collection/agglomeration apparatus 14.

TABLE 2 Object Filtration particle wind velocity Type Principle size [μm] [m/s] Evaluation Corrugated Inertial 20≤   2 to 4.5 x mist removal impaction apparatus Wire mesh Interception 3 to 20  1.3 to 3.5 ∘ Filtration Brownian ≤3 0.03 to 0.2 x fabric diffusion (candle filter)

FIG. 3 is a schematic view of mist bloating according to the present invention. In addition, in FIG. 3, the lower side of the figure is the gas flow downstream side and the upper side of the figure is the gas flow upstream side. As illustrated in FIG. 3, in a case where a wire mesh 70 is used as the mist collection/agglomeration apparatus 14, particles of the SO₃ mist 50 adhered to the surface of the wire mesh 70 come into contact with each other and agglomerate, thereby forming the agglomerated SO₃ mist 51. In addition, the agglomerated SO₃ mist 51 flows downward along the wire mesh 70, and falls dropwise from the vicinity of the lower end portion of the wire mesh 70 as a mist drain to be discharged from the mist collection/agglomeration apparatus 14.

Here, in a case where the filtration wind velocity of the flue gas 12B is set to be equal to or higher than a wind velocity value regarded as the limit during typical mist collection, downflow and falling of the agglomerated SO₃ mist 51 are partially disturbed and re-scattering thereof from the upper surface side of the wire mesh 70 occurs, resulting in a reduction in the mist removal efficiency. Therefore, the filtration wind velocity regarded as the limit in the wire mesh type mist removal apparatus is, for example, 2.5 to 5 m/s (varies depending on the mist load in the gas and the wire mesh type).

FIG. 4 is a diagram showing an example of the relationship between the filtration wind velocity and the mist removal ratio. FIG. 5 is a diagram showing an example of the relationship between the filtration wind velocity, the mist adhesion ratio, and the mist re-scattering ratio.

In the example shown in FIG. 4, the mist removal ratio increases as the filtration wind velocity increases until the filtration wind velocity approaches, for example, 2.8 m/s. However, when the filtration wind velocity exceeds 2.8 m/s, re-scattering of the SO₃ mist collected by the wire mesh 70 and aggregated starts.

Furthermore, as shown in FIG. 5, the re-scattering ratio of the SO₃ mist adhered to the wire mesh 70 increases from the vicinity of the critical filtration wind velocity (for example, 2.8 m/s).

In a region of higher than the critical filtration wind velocity (for example, 2.8 m/s), the amount of the re-scattering agglomerated SO₃ mist 51 increases as the filtration wind velocity increases, and accordingly, as shown in FIG. 4, the mist removal ratio of the wire mesh (mist removal apparatus) 70 decreases.

However, agglomeration of the mist adhered to the wire mesh 70 occurs, and the particle size of the re-scattering mist is greater than the particle size of the inflow mist. Therefore, the re-scattering agglomerated SO₃ mist 51 becomes the nucleus and is further bloated in the CO₂ absorption tower 16, so that the mist recovery ratio of the demister which is the mist collection unit 16C installed at the outlet of the CO₂ absorption tower 16 is improved. As a result, the amount of mist accompanying the CO₂ absorbent and scattering to the outside of the system can be reduced.

As described above, as the filtration wind velocity (V) of the flue gas in the mist collection/agglomeration apparatus 14, a wind velocity (for example, V>2.5 m/s) which exceeds the filtration critical wind velocity (for example, 2.5 m/s) for the mist collection or a wind velocity of 1.2 to 1.5 times the filtration critical wind velocity is preferable.

Here, since the filtration wind velocity in the mist collection/agglomeration apparatus 14 varies depending on the mist load in the flue gas and the wire mesh type, in an actual apparatus, it is preferable to set the wind velocity to a wind velocity of 1.2 to 1.5 times the filtration critical wind velocity after the critical filtration wind velocity is determined.

According to this example, by the mist collection/agglomeration apparatus 14, the agglomerated SO₃ mist 51 which is agglomerated and bloated mist is formed by causing the particles of the SO₃ mist 50 contained in the flue gas 12B from the desulfurization apparatus 13 to be bonded together and have bloated particle sizes, the agglomerated SO₃ mist 51 is caused to re-scatter and be introduced to the CO₂ absorption unit 16A side which removes CO₂ by bringing the CO₂ absorbent on the gas flow downstream side into contact therewith. Thereafter, in the CO₂ absorption unit 16A, when CO₂ contained in the flue gas is removed by being brought into contact with the CO₂ absorbent, the CO₂ absorbent bloated mist 53 bloated by the CO₂ absorbent being absorbed by the agglomerated SO₃ mist 51 as the nucleus is formed, and the CO₂ absorbent bloated mist 53 can be collected by the mist collection unit 16C. Accordingly, when the treated flue gas from which CO₂ has been removed is discharged to the outside of the system, entrainment of the CO₂ absorbent can be significantly suppressed.

Example 2

A flue gas treatment system according to Example 2 will be described below. In this example, the mist collection/agglomeration apparatus 14 of Example 1 is caused to bloat mist through charging. FIG. 6 is a schematic view of mist bloating caused by charging. As illustrated in FIG. 6, in this example, SO₃ mist is charged by using a discharge electrode 102 provided with a high voltage power supply 101.

In this example, as the mist collection/agglomeration apparatus 14 using charging, there are the discharge electrode 102 for charging the SO₃ mist 50 and a low pressure loss filter 120 which is grounded 111. The SO₃ mist 50 charged by using electrostatic force electrically neutralizes the adhered SO₃ mist in order to prevent repulsion due to the electrostatic force between particles of the mist adhered to the low pressure loss filter 120.

In this example, by using the electrostatic force, compared to the case of using only the inertia/interception/diffusion effect (Brownian effect), the collection performance of the SO₃ mist is improved.

Furthermore, by using the electrostatic force, compared to the case of using only the inertia/interception/diffusion effect (Brownian effect), the mist collection/agglomeration apparatus 14 can be downsized.

FIG. 7 is a diagram of the relationship between the SO₃ mist concentration in the inlet flue gas 12D of the CO₂ absorption unit 16A and the amine concentration in the gas released from the outlet of the mist collection unit 16C of the CO₂ absorption tower 16.

In the case of this example, it was confirmed that by providing the mist collection/agglomeration apparatus 14 using charging, a reduction in the amine concentration in the gas released from the outlet of the mist collection unit 16C of the CO₂ absorption tower 16 is smaller than that of a comparative example.

REFERENCE SIGNS LIST

10 flue gas treatment system

11 boiler

12A to 12F flue gas

13 desulfurization apparatus

14 mist collection/agglomeration apparatus

15 flue gas cooling tower

16 CO₂ absorption tower

16A CO₂ absorption unit

16B washing unit

16C mist collection unit

17 absorbent regeneration tower

18 CO₂ recovery apparatus

50 SO₃ mist

51 agglomerated SO₃ mist

53 CO₂ absorbent bloated mist 

The invention claimed is:
 1. A flue gas treatment system comprising: a desulfurization apparatus which removes sulfur oxides in flue gas from a boiler, which is configured to generate SO₃ mist from a portion of SO₃ gas in the flue gas; a mist collection and agglomeration apparatus which is provided on a gas flow downstream side of the desulfurization apparatus and introduces the flue gas from the desulfurization apparatus, which is configured to form agglomerated SO₃ mist by causing particles of the SO₃ mist contained in the flue gas to be bonded together and have particle sizes bloat; wherein the mist collection and agglomeration apparatus is configured to re-scatter the agglomerated SO₃ mist therefrom in a case a flow velocity of the flue gas exceeds a critical filtration wind velocity for mist collection; a flue gas cooling tower which is provided on the gas flow downstream side of the mist collection and agglomeration apparatus and introduces the flue gas from the mist collection and agglomeration apparatus and cools the flue gas, which is configured to form dilute sulfuric acid mist from the agglomerated SO₃ mist contained in the flue gas by absorbing water vapor; and a CO₂ recovery apparatus which is provided on the gas flow downstream side of the flue gas cooling tower, comprising: a CO₂ absorption tower including a CO₂ absorption unit which introduces the flue gas from the flue gas cooling tower and removes CO₂ contained in the flue gas by being brought into contact with a CO₂ absorbent and a washing unit which is provided on the gas flow downstream side of the CO₂ absorption unit and washes the flue gas, which are configured to form CO₂ absorbent bloated mist from the dilute sulfuric acid mist contained in the flue gas by absorbing water vapor and CO₂ absorbent vapor, and a mist collection unit which is provided on a gas flow downstream side of the washing unit and is configured to collects the CO₂ absorbent bloated mist; and an absorbent regeneration tower which introduces the CO₂ absorbent having absorbed CO₂ in the CO₂ absorption tower therefrom and recovers CO₂ from the CO₂ absorbent by releasing CO₂ therefrom to regenerate the CO₂ absorbent.
 2. The flue gas treatment system according to claim 1, wherein the mist collection and agglomeration apparatus includes a wire mesh.
 3. The flue gas treatment system according to claim 1, wherein the mist collection and agglomeration apparatus includes a discharge electrode for charging the SO₃ mist and a low pressure loss filter grounded. 